Effect of alignment mark depth on alignment signal behavior in advanced lithography

Transcription

1 Journal of Engineering Research and Education Vol. 5 () 7- Effect of alignment mark depth on alignment signal behavior in advanced lithography 1 Normah Ahmad, 1 Uda Hashim, Mohd Jeffery Manaf, Kader Ibrahim Abdul Wahab 1 Institute of Nanooelectronic Engineering, Universiti Malaysia Perlis, Kangar, 1 Perlis. uda@kukum.edu.my SilTerra Malaysia Sdn. Bhd.Kulim Hi-Tech Park, 9 Kulim, Kedah. normah_ahmad@silterra.com ABSTRACT Finding a robust alignment strategy is one of the key evaluations in defining photolithography process. Alignment is a process to determine how the current pattern is placed on the wafer. Alignment is done by an optical system, which means that it is dependable on the quality of the alignment signal to determine the correct orientation. Alignment signal is generated by alignment mark, a diffraction grating structure (trench and line structure) printed on wafer. Hence, the processing steps can possibly affect the properties of alignment mark. The alignment mark depth (trench depth) can be varied due to the nature of processing. According optics, a light optical path variation may lead to a destructive interference, which is not good.. Keywords: Alignment signal, lithography, Alignment mark INTRODUCTION The challenge in photolithography process is to print the desired feature size and to align the feature correctly to the subsequent layer [Rigorous Coupled wave analysis of FE of line wafer alignment marks]. It is important to ensure that the new pattern is overlaid the previous pattern correctly. Designing a robust alignment strategy is difficult and time consuming since the scanner alignment system is designed to work with an ideal profile of alignment mark. There have several factors can affect the alignment signal significantly which is size of alignment mark, polarity, processing steps, type of alignment system, refractive index, reflectivity, grain structure, and surface roughness [Laser Alignment Modeling Using Rigorous Numerical Solution]. This paper will concentrate on how processing steps affect the alignment signal behavior.

2 Normah Ahmad et al. / Effect of alignment mark depth on alignment Previous work by [Experimental and simulation studies of alignment marks] stated that alignment signal is a strong function of trench depth. From [Segmented Alignment Mark Optimization and Signal Strength Enhancement for Deep Trench Process], for a rectangular grating profile, the intensity is given by; I πr = sin ( ) I R π r + 1 sin α ( ) (1) where I: the incident radiation intensity R: the reflection coefficient. r: the ratio between area of the higher and lower lines of the phase grating. α: optical phase difference between light reflected from the higher and lower lines of the grating. Besides the perfection of alignment system, stray light can leak into the alignment system. If the mark depth is small, or the duty cycle is low, the relationship between the alignment signal intensity and the stray light intensity becomes critical. Too much stray light in the alignment system leads to unreliable alignment results. To small mark depth also makes the alignment mark visibility is lesser. Reducing the visibility weakens alignment signal strength, increases the probability of overlay error, alignment system failure, and rework [Optical Flatness and Alignment Mark Contrast in Highly Planar Technologies]. The behavior of alignment signal over a range of mark depth is depended on the laser wavelength. Theoretically, a reduction in signal strength occurred when the alignment mark depth value is in range of multiple lambda/ [Experimental and Simulation Studies of alignment mark]. According physics theory; this is a point where a destructive interference occurred. ALIGNMENT MARK FORMATION FOR CONTACT LAYER (CONTACT MARK) Alignment mark is printed on the wafer. This means that it is going through exactly the same process as the rest of the circuit structure. The first step in order to produce a contact structure is to deposit oxide material at certain required thickness. Since the deposition process does not produce a flat surface, hence, planarization process is required. Then, the oxide material will go through a

3 Journal of Engineering Research and Education Vol. 5 () 7- patterning and etch process to produce the desired shape. Figure 1 illustrate the post lithography and etch process alignment mark. Figure 1. Post lithography and etch contact alignment mark. After that, tungsten material will be deposited. Tungsten material has to be completely removed from oxide upper surface. Tungsten is removed through chemical mechanical planarization (CMP) process. Contact hole will be completely filled by tungsten due to its small feature size. However, alignment mark trench won t be completely filled since its trench size is much larger than the tungsten deposition thickness (Figure ). Figure 3 illustrates the alignment mark profile after tungsten planarization process. Figure. Alignment Mark Profile after Tungsten Deposition. Figure 3. Post Tungsten CMP Profile. 9

4 Normah Ahmad et al. / Effect of alignment mark depth on alignment Finally, metal/aluminum layer will be deposited on the existing structure through physical deposition process (PVD). Usually, the asymmetric profile becomes significant after this process. Figure shows the possible profile after metal layer is deposited. This is final profile of contact alignment mark. The reflected signal behavior during metal1 layer alignment depends on this profile Figure. Post Aluminum/Metal Layer Deposition Profile From description above, alignment mark depth variation is influenced by final oxide thickness, tungsten deposition thickness, and tungsten planarization time. RESEARCH METHODOLOGY Based from the conclusion of the previous section, an experiment was designed in order to investigate how various processing steps affect the alignment mark depth and eventually alignment signal performance. Each of the alignment mark will be evaluated for each particular condition. Split Table 1. Process Split based from DOE Final ILD CMP Thickness Tungsten Deposition Thickness Tungsten Planarization Time A 9 3 B 7 C D 7 3 E 7 F 7 3 G 9 3 H 9 I 9 1

5 Journal of Engineering Research and Education Vol. 5 () 7- Oxide measurement will be measured right after ILD planarization and after tungsten planarization process. This measurement is important so that the theoretical alignment mark depth can be calculated. In this experiment, three main types of alignment mark were used which are AH3, AH53, and AH7. Please refer to table for the details of alignment mark used in this experiment. Alignment signal strength behavior is represented by Wafer Quality () parameters. Table. Alignment Mark Type and their optimized light order recipe Mark Type Subtype Architecture Optimized light Recipe Type AH3 AH3-CON-PRI No Poly Block AH3-CONPLB-PRI Poly Block Third Order Light AH53-CON-PRI No Poly Block AH53-CONPLB-PRI Poly Block AH53 AH53-CVV-NPLB No Poly Block Fifth order Light AH53-CVV-SCR Poly Block AH53S-CVV-SCR Poly Block AH7-CON-PRI No Poly Block AH7 AH7-CONPLB-PRI Poly Block AH7-CVV-SCR Poly Block Seventh Order Light AH7DC-CVV Poly Block RESULT AND DISCUSSION Alignment mark is a reflected diffraction grating structure. Hence, the signal behavior generated by alignment mark follows the diffraction rules. Equation 1 suggest that there have four factors may affect the reflected light intensity from alignment mark, which is incident radiation intensity, the reflection coefficient, the ratio between area of the higher and lower lines of the phase grating, and optical phase difference between light reflected from the higher and lower lines of the grating. The optical phase is interpreted as alignment mark depth (Figure 5). 11

6 Normah Ahmad et al. / Effect of alignment mark depth on alignment Figure 5. Cross sectional view of alignment mark. Yellow mark shows an alignment mark depth. In this work, alignment mark depth is calculated based from oxide/metal stacking. (a) (b) Figure. Alignment Mark Structure (a) Poly Block Alignment Mark Structure (b) No Poly Block Alignment Mark Structure. Depending on whether the alignment mark structure consists of poly block or not (refer to Figure 7 for stacking details), the alignment mark depth is calculated based from equation and equation 3. 1

7 Journal of Engineering Research and Education Vol. 5 () 7- Theoretical Alignment Mark Depth = Final Oxide Thickness (post CMP Inter Layer Dielectric (ILD)/Inter Layer Metal (IMD) Tungsten Deposition Oxide Loss during Tungsten CMP Poly Block Thickness (if exist) () Oxide Loss during Tungsten CMP = Final Oxide Thickness (post CMP Inter Layer Dielectric (ILD/Inter Layer Metal (IMD) Final Oxide Thickness after Tungsten CMP (3) (a) (b) Figure 7. AH3 Alignment Mark Type vs. Mark Depth. (a) AH3-CON-PRI (b) AH3 CONPLB-PRI. 13

8 Normah Ahmad et al. / Effect of alignment mark depth on alignment (a) (b) (c) 1

9 Journal of Engineering Research and Education Vol. 5 () (d) (e) Figure. AH53 Alignment Mark Type vs. Alignment Mark Depth (a) AH53-CON-PRI (b) AH53-CONPLB-PRI (c) AH53-CVV-NPLB (d) AH53-CVV-SCR (e) AH53S- CVV-SCR 15

10 Normah Ahmad et al. / Effect of alignment mark depth on alignment (a) (b) (c) 1

11 Journal of Engineering Research and Education Vol. 5 () (d) Figure 9. AH7 Alignment Mark Type vs. Alignment Mark Depth (a) AH7-CON-PRI (b) AH7-CONPLB-PRI (c) AH7-CVV-SCR (d) AH7DC-CVV Figure 7 shows the signal strength trend for AH3 mark plotted with respect to theoretical alignment mark depth. Basically, there is no trend over the alignment mark depth variation for both AH3 mark since the signal strength is almost zero. Excluding AH53-CONPLB-PRI and AH53-CVV-SCR mark, the rest of the AH53 alignment mark shows a signal variation trend as the alignment mark depth also varied (Figure ). From t-test, their signal variations are significant. Even though there is a significant difference signal variation for AH53-CVV-SCR as shown in Figure 7d, there is no physical trend observed on the Figure 7d graph. From Figure, all AH7 mark type except AH7-CVV-SCR shows a significant signal variation (From t-test) with respect to the alignment mark depth variation. AH7-CVV-SCR mark have an overall zero signal strength throughout the mark depth variation. 17

12 Normah Ahmad et al. / Effect of alignment mark depth on alignment Table 3.Theoretical Alignment Mark Depth where the signal starts to degraded/increasing compared to signals at other mark depth. at Alignment Mark Type point where alignment signal degraded/increasing AH3-CON-PRI - AH3 CONPLB-PRI - AH53-CON-PRI AH53-CONPLB-PRI - AH53-CVV-NPLB AH53-CVV-SCR - AH53S-CVV-SCR AH7-CON-PRI AH7-CONPLB-PRI AH7-CVV-SCR - AH7DC-CVV Generally, the alignment signals starts to degrade at 913Å to 519Å range and 913Å 37 Å (Table 3). All mark given in Table 3 is only theoretical values calculated based from process stacking. More or less, this behavior are truly reflected the actual situation during alignment. CONCLUSION From the results, there is a degradation of signal strength over a range of alignment mark depth. Unfortunately, alignment mark depth could not be controlled since it is dependent on process characteristics such as oxide thickness and over polish time. This parameter cannot be simply changed since it may impact the overall production yield. The best that can be done is to find a selection of alignment mark, which is less sensitive to the alignment mark depth variation. ACKNOWLEDGEMENT The authors would like to thanks Silterra (M) Sdn Bhd and their employees for supporting this research 1

13 Journal of Engineering Research and Education Vol. 5 () 7- REFERENCES [1] Christopher Palmer, Diffraction Grating Handbook, Thermo RGL Grating Lab. [] Levinson, H. J. (1). Principles of Lithography SPIE Press. pg 1-3 [3] ASML. (1999) ATHENA Knowledge Sharing, Application Note, pg. [] Yuanting Cui, A. S., Sean Louks. (). Fine Tune W-CMP Process with Alingment Mark Selection for Optimal Metal Layer and Yield Benefits. Paper presented at the SPIE [5] ASML. () Process Effects on Alignment: Tungsten CMP/Aluminum PVD, Application Note, pg [] Diffraction; Thin Film Interference (1999). from http//physics.bu.edu. [7] Trench Filling by Physical Vapor Deposition (). from [] Jamas R. Sheats, B. W. S. (199). Microlithography, Science and Technology. Marcel Dekker Inc. [9] S. Wolf, R. N. T. (). Silicon Processing for the VLSI Era Volume 1- Process Technology (nd Edition ed.): Lattice Press. [1] Pantazis Mairoulis, J. M. D. (199). Geogmetrical Optics and Optical Design Oxford University Press. [11] Rouchouze, E., Darracq, Jean Micheal, Gemen, Jack (1997). CMP- Compatible [1] Alignment Strategy Paper presented at the SPIE. [1] Thomas V. Pisto, R. J. S. (). Rigorous Electromagnetic Simulation of Stepper Alignment. 19

14 Normah Ahmad et al. / Effect of alignment mark depth on alignment